Plasma display panels (PDP), as used in direct television and high definition television applications, conventionally use di-valent europium activated barium magnesium aluminate (BAM) phosphor as a blue emitting component due to its availability and high quantum efficiency. However, compared with other phosphors such as Eu.sup.3+ (red) and Tb.sup.3+ (green) activated yttrium, gadolinium borate-based phosphors or Mn activated zinc silicate, BAM exhibits a wide spectrum of emission with poor color purity and low lifetime under a VUV flux.
Lifetime of a plasma display is directly related to the performance of phosphors used in the display. Therefore, lifetime of phosphors is of concern in selecting suitable phosphors. Displays for consumer and commercial applications should exhibit lifetimes on the order of 30,000 hours of operation. Therefore, considerable effort has been made to develop new phosphors to replace BAM and to provide improved performance characteristics.
Tm.sup.3+ activated lanthanum phosphate is one candidate that has been investigated by Applicant herein and is described in the parent Application hereof, i.e. U.S. patent application Ser. No. 09/110,500, now U.S. Pat. No. 5,989,454. The aforesaid phosphor exhibits two narrow peaks in the UV region (340 to 370 nm) and a visible peak at 452 nm. The phosphor's brightness in the visible region, however, is not able to meet current brightness requirements.
Combinations of UV excitable light-emitting phosphors and UV light emitting phosphors are known in the art. U.S. Pat. No. 5,747,100 to Peterson teaches a method of making a low voltage phosphor for field emission displays by forming a diffusion barrier of UV-emitting material on the UV excitable phosphor. In lamp applications, UV emitting phosphors are blended with UV excitable phosphors to improve the performance of broad spectrum lamps. For example, U.S. Pat. No. 4,891,550 to Northrop et al. describes a phosphor blend having four different phosphors covering the visible and partially the UV spectra (5%-8%). The object of the phosphor blend is to produce UV light in a manner close to sunlight.
U.S. Pat. No. 5,801,483 to Watanabe et al. describes a phosphor blend for a fluorescent lamp which converts the ultraviolet rays from the fill gas into visible light and UV radiation in the 320-410 nm range. The luminescent compound is a blend of red emitting trivalent europium activated yttrium oxide, blue emitting barium magnesium aluminate activated by divalent europium, green emitting lanthanum cerium phosphate activated by trivalent terbium and UV emitting phosphors of either barium silicate activated by divalent lead or divalent europium activated strontium magnesium pyrophosphate, or trivalent cerium activated yttrium phosphate.
Most of the work reported on lanthanum phosphate based phosphors has been related to fluorescent lamp applications as an efficient green phosphor and the performance of the phosphor therein. Development of terbium and cerium activated lanthanum phosphate is well documented in numerous patents. Different methods of preparation and the introduction of various impurities have been tried in attempts to improve the life and performance of the lamp.
U.S. Pat. No. 3,211,666 to William A. McAllister discloses use of lanthanum phosphate activated with various rare earths, for high pressure mercury vapor lamps and CRTs. Synthesis of particular phosphors was made by mixing one mole of lanthanum oxide with two moles of ammonium dihydrogen ortho phosphate and 0.08 mole of rare earth oxide and fired in a nitrogen atmosphere at a temperature 1100.degree. C. for 90 minutes.
In U.S. Pat. No. 3,507,804, rare earth (Ce, Tb, Eu, Tm, Yb, Pr, Nd) activated Y,Gd,La phosphate was synthesized by reacting respective solutions with phosphoric acid solution. The dried precipitates were fired in air at 1150-1200.degree. C. for three to four hours.
PCT patent WO 99/21938 describes the preparation of lanthanum phosphate comprising thulium from respective salts and phosphoric acid in presence of flux at 1000.degree. C.
U.S. Pat. No. 4,423,349 to Nakajima et. al. describes two methods of synthesizing the above phosphor. In the first method, lanthanide carbonates are reacted with phosphoric acid at 75.degree. C. and then calcinated at 1150.degree. C. for 75 minutes. In the second method, coprecipitated lanthanide oxalates are oxidized to a single phase lanthanide oxide at 800.degree. C. Diammonium phosphate is mixed with the oxide and fired at 1200.degree. C. Boron oxide or ammonium borate is also added before calcination to enhance the reaction and also improve the brightness.
U.S. Pat. No. 5,091,110 to Albert et.al. discloses a method of making lanthanum cerium terbium phosphate phosphor in a two step process. The method comprises formation of an aqueous solution of lanthanide nitrates and an aqueous solution of diammonium phosphate and combining both to coprecipitate a lanthanum terbium cerium phosphate followed by firing the mixture at higher temperatures to form the phosphor. Boron phosphate is used as the phosphate source because it is stable at elevated temperatures (see U.S. Pat. No. 5,132,042). Lithium carbonate is also used as a flux forming compound to improve the solubility of the lanthanide phosphate in the boron oxide solution formed during the process (see U.S. Pat. No. 5,154,852).
Terbium, cerium activated lanthanum phosphate is also prepared by reacting monoammonium phosphate solution and respective rare earth nitrate solutions (U.S. Pat. No. 5,340,556 to Collin et al.). The resultant powder is calcined at 900.degree. C. in air or in a non-reducing atmosphere to obtain a phosphor with 250 nm compact aggregates. From XRD analysis, it is found that the resultant phosphor powder has monoclinic crystal structure. Small size phosphor particles could be prepared by adding excess boric acid and lithium carbonate as a flux in the starting mixture before firing (see U.S. Pat. No. 5,651,920 to Chau et al.).
U.S. Pat. No. 5,746,944 to Braconnier et al. disclose a lanthanum/cerium/terbium mixed green phosphor that is directly precipitated by reacting a first solution of soluble lanthanum, cerium and terbium salts with a second solution containing phosphate ions.
HDTV and similar type display devices should have high resolution and high brightness to meet expected performance. This can be achieved currently only with thin phosphor screens consisting of very small phosphor particles (0.5-2 microns) in a close rib structure. Screens with small particles have higher packing density and also need lesser binder content. It is known that terbium and cerium activated lanthanum phosphate have high quantum efficiency, better stability at operating temperatures and long lifetime, particularly under 254 nm UV excitation (compact fluorescent lamps). However, very limited information is available on the preparation and luminescent studies on thulium activated lanthanum phosphate phosphors.